I say the mark III(though i never rode it as it was just phased out when i was born). The reason being is that it had the looks of the original with more window space and cars(for some odd reason i don't like monorails with four cars per train)
I like the Mark V. I remember the Mark III since they were still operating when I was a kid and I thought they were louder than the Mark V since they had heavier bodies. I just love the lear jet look the Mark Vs have, too.
Mark V. I love, love, love the look of these trains. They look clean and sleek, and you just can not beat that Lear jet nose.
I've been going to the park since... well... for a long time, so I've seen a few different "Marks", but this one just does it for me. It looks good no matter where it is on the beam; Tomorrowland, Fantasyland, Downtown Disney, or DCA. I don't think any of the earlier designs could say that, even as cool as they were.